Many electronic components such as power converters utilize wires or cables to carry voltage and/or current from one point in a circuit to another. Generally, these wires may be constructed from a conductive material (e.g., copper), which has a resistance to current flow that may contribute to power loss (sometimes called “copper loss”) in an electronic component. Therefore, it may be desirable to minimize the power loss in the conductors in order to provide more efficient components.
Generally, the resistance of a conductor at DC (0 Hertz) depends on its cross sectional area. A conductor with a larger cross sectional area has a lower resistance than a conductor with a smaller cross sectional area. For AC current, a phenomenon known as the “skin effect” causes that resistance to increase substantially with increasing frequency of current.
The skin effect is the tendency of an AC electric current to distribute itself within a conductor such that the current density (i.e., current per cross-sectional area) near the surface of the conductor is greater than at its core. In other words, the current tends to flow at the “skin” of the conductor. The skin effect is due to eddy currents formed by the AC current. The decline in current density verses depth from the surface is often quantified by a measure of the distance from the surface of the conductor over which the current density decays to 1/e (or about 37%) of its value at the surface. This measure is generally referred to as “skin depth.”
For low frequencies, the skin effect may be negligible. For AC current at frequencies high enough that the skin depth is small compared to the conductor diameter, the skin effect causes most of the conduction to happen at the conductor's surface. At high enough frequencies, the interior of a large conductor does not carry much current. As an example, at 60 Hz, the skin depth of a copper wire is about 0.3 inches (8 mm). At 60 kHz, the skin depth of copper is about 0.01 inches (0.254 mm). At 6 MHz, the skin depth is about 0.001 inches (25.4 μm). As can be appreciated, conductors larger than a few skin depths do not conduct much current near their interior axis, so that material isn't used effectively.
A type of cable called bunched wire or litz wire (from the German litzendraht, braided wire) may be used to mitigate the skin effect for current with relatively high frequencies, such as a few kilohertz, a few megahertz, or more. A cross-sectional view of a litz wire 10 is shown in FIG. 1. The litz wire 10 includes a number of insulated wire strands 15 woven together in a pattern (e.g., twisted, braided, or the like), so that the overall magnetic field acts substantially equally on all the strands and causes the total current to be distributed equally among them. Further, the radius of the individual strands may be chosen to be less than a skin-depth for a particular application, so the individual strands do not suffer an appreciable skin effect loss. The individual strands generally include an outer layer of insulation 16 to electrically insulate them from each other, such that the strands in a bundle are not shorted together. Further, the entire bundle of strands 15 may include an outer insulation layer 25.
Litz wire may be used in the windings of high-frequency transformers, to increase their efficiency by mitigating both skin effect and another phenomenon referred to as proximity effect, which is caused by an interaction of magnetic fields between multiple conductors. The weaving or twisting pattern of litz wire may be selected so that individual wires will reside for short intervals on the outside of cable and for short intervals on the inside of the cable, which may allow the interior of the litz wire to contribute to the cable's conductivity.
Undesirably, standard litz wire may reduce the effective copper area (relative to a solid conductive wire) since the insulation on each of the smaller individual wire strands consumes a significant amount of the net cross-sectional area of the litz wire. Furthermore, the packing of equally sized round strands next to each other leaves a relatively large percentage of the cross-sectional area taken by air space. Standard litz wire therefore results in relatively small amounts of copper in the cross-sectional area of the wire compared with standard wire of the same cross-sectional area.